docs: set up DocFX documentation with GitHub Pages deployment

[tl-Roberto-Mancinelli]

Summary

This PR sets up comprehensive DocFX documentation for the TrueLayer .NET SDK, including:

• Complete documentation structure with articles covering:
  • Installation and authentication
  • Payments, payouts, and mandates
  • Provider management and merchant accounts
  • Error handling and configuration
• GitHub Actions workflow for automatic deployment to GitHub Pages
• Custom styling with TrueLayer branding
• API reference documentation generation from XML comments

Documentation Structure

• Getting Started: Installation, authentication, error handling
• Core Features: Payments, payouts, mandates, providers, merchant accounts
• Advanced Topics: Multiple clients, custom configuration
• API Reference: Complete API documentation from code

Key Improvements

• Aligned status terminology with official TrueLayer documentation
• Added references to the MVC Example throughout
• Used proper constants (e.g., ReleaseChannels.GeneralAvailability)
• Server-side filtering examples for providers
• Comprehensive error handling examples

Preview Documentation Locally

To preview the documentation changes locally:

• Restore dotnet tools dotnet tool restore
• Build the documentation dotnet docfx docs/docfx.json
• Serve the documentation locally dotnet docfx serve docs/_site

Then open your browser to http://localhost:8080 to view the documentation.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud